1. Field of the Invention
The present invention relates to a variable reactance circuit which combines an electrical adjusting means to an integrated circuit for an microwave or millimeter wave band (to be referred to as an MIC hereinafter). More particularly, the present invention relates to a variable matching circuit which incorporates the variable reactance circuit.
2. Description of the Related Art
In a circuit device used in an ultra-high-frequency band such as a microwave band or a millimeter wave band, when circuit elements are not preferably matched with each other, a satisfactory circuit function cannot be obtained. However, as a frequency for the signal input to the circuit device is increased, variations in characteristics of the circuit elements and circuit losses tend to increase. For this reason, in a monolithic MIC which includes fixed elements to which post adjustment cannot be performed, a problem is posed, i.e., a low yield disables practical applications of the monolithic MIC.
To overcome such problems, a conventional technique which incorporates a hybrid MIC to which post adjustment can be performed via bonding is commonly used. However, since the hybrid MIC is a distributed constant circuit constituted by micro-strip lines, the hybrid MIC requires a magnitude which is much higher than a lumped constant, where the hybrid MIC is not suitable for mass production. For this reason, an adjusting mechanism for a monolithic MIC is in demand.
Accordingly, an adjusting mechanism and a short-circuiting mechanism are considered. Such an adjusting mechanism uses the bias voltage dependency of the input impedance of a field effect transistor (FET), and such means for selectively short-circuiting spiral inductor lines use a switching mechanism to adjust inductance values. However, in the adjusting mechanism an amplifying FET cannot be used under a bias voltage condition which is optimally suitable for amplification. For that reason degradation of circuit performance is posed as a problem. In addition, undesired losses caused by the resistors of the spiral inductors themselves are experienced in the switching mechanism.
A mechanism for adjusting a capacitance by a varactor, a mechanism for adjusting a reactance by the resonance of a varactor, and a spiral inductor are therefore considered to overcome such problems. However, the adjusting mechanism using the varactor has the following problems: the variable range of the varactor is limited, and a bias voltage for varying the capacitance must be high.
FIG. 6A shows an ideal amplifying circuit designed by conventionally known reactance elements, each having a loss. This amplifying circuit comprises input and output matching circuits, each constituted by an amplifying FET 51, reactances 52 and 53 which are respectively series-connected to the gate and drain of the FET 51, and parallel reactances 54 and 55.
As described above, although the matching circuit requires reactance elements, the types of reactance elements which can be actually used in a monolithic MIC are limited. More specifically, a spiral inductor only serving as a lumped-constant inductive reactance element has a serious problem in that a loss caused by the resistance of a line cannot be neglected. Therefore, the matching circuit cannot be formed into a monolithic MIC without any change. For this reason, a hybrid MIC having a distributed constant arrangement which can realize an arbitrary reactance with a low loss is popularly used.
FIG. 6B shows an amplifying circuit constituted by the above hybrid MIC. Strip lines 62 and 63 which are connected to an amplifying FET 61 constitute an input matching reactance circuit. Strip lines 64 and 65 constitute an output matching reactance circuit. In addition, in the hybrid MIC, the input and output matching circuits can be post-adjusted by bonding adjusting patterns 631 and 651. Therefore, the degradation of performance caused by variations in characteristic of the FET 61 can be corrected. However, a physical adjusting mechanism which operates based on bonding is not suitable for mass production, so that it cannot be applied to the monolithic MIC. For this reason, an electrical adjusting means capable of automatically performing the adjustment is required.